Oh please! Don't get all uppity about her reaction now! Think about it from her side and have some compassion. You want what you want and have been mulling this over for a while, hoping for a positive response -- but for her, you literally dropped a fucking bomb in her lap!
She's not blackmailing you -- she's being honest in telling you what she can and cannot live with AND I am sure her words are fueled by extreme emotion and possibly a sense of not being enough for you, rejection, hurt, feeling betrayed -- all of that. That is often what it seems people take it to mean when a partner wants to go poly.
You only told her today. She may need some time to process this and not take it personally. Stop going over the conversation in your mind and occupy yourself with other things. Look at her, really look at this wonderful woman you love and see how scared she is of everything she counted on falling apart. Let her know you love her. Without her knowing that, if she ever agrees to accept poly, you won't have a strong foundation upon which to build. Baby steps!